Fabrication of MOS device with schottky barrier controlling layer

ABSTRACT

Fabricating a semiconductor device includes: forming a gate trench in an epitaxial layer overlaying a semiconductor substrate; depositing gate material in the gate trench; forming a body; forming a source; forming an active region contact trench that extends through the source and the body into the drain; forming a Schottky barrier controlling layer in the epitaxial layer in bottom region of the active region contact trench; and disposing a contact electrode within the active region contact trench.

CROSS REFERENCE TO OTHER APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/005,166 entitled MOS DEVICE WITH SCHOTTKY BARRIER CONTROLLING LAYERfiled Dec. 21, 2007, which is incorporated herein by reference for allpurposes, which is a continuation in part of U.S. patent applicationSer. No. 11/900,616 entitled POWER MOS DEVICE filed Sep. 11, 2007, nowU.S. Pat. No. 7,605,425, which is incorporated herein by reference forall purposes, and which is a continuation of U.S. patent applicationSer. No. 11/056,346 entitled POWER MOS DEVICE filed Feb. 11, 2005, nowU.S. Pat. No. 7,285,822, which is incorporated herein by reference forall purposes.

BACKGROUND OF THE INVENTION

Power MOS devices are commonly used in electronic circuits. Depending onthe application, different device characteristics may be desirable. Oneexample application is a DC-DC converter, which includes a power MOSdevice as a synchronous rectifier (also referred to as the low side FET)and another power MOS device as a control switch (also referred to asthe high side FET). The low side FET typically requires a smallon-resistance to achieve good power switch efficiency. The high side FETtypically requires a small gate capacitance for fast switching and goodperformance.

The value of a transistor's on-resistance (R_(dson)) is typicallyproportional to the channel length (L) and inversely proportional to thenumber of active cells per unit area (W). When choosing a value forR_(dson), consideration should be given to the tradeoff betweenperformance and breakdown voltage. To reduce the value of R_(dson), thechannel length can be reduced by using shallower source and body, andthe number of cells per unit area can be increased by reducing the cellsize. However, the channel length L is typically limited because of thepunch-through phenomenon. The number of cells per unit area is limitedby manufacturing technology and by the need to make a good contact toboth the source and body regions of the cell. As the channel length andthe cell density increase, gate capacitance also increases. Lower devicecapacitance is preferred for reduced switching losses. In someapplications such as synchronous rectification, the stored charge andforward drop of the body diode also result in efficiency loss. Thesefactors together tend to limit the performance of DMOS power devices.

It would be desirable if the on-resistance and the gate capacitance ofDMOS power devices could be reduced from the levels currentlyachievable, so that the reliability and power consumption of the powerswitch could be improved. It would also be useful to develop a practicalprocess that could reliably manufacture the improved DMOS power devices.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the followingdetailed description and the accompanying drawings.

FIGS. 1A-1F illustrate several double-diffused metal oxide semiconductor(DMOS) device embodiments.

FIG. 2 is a diagram illustrating a buck converter circuit example.

FIG. 3 is a flowchart illustrating an embodiment of a fabricationprocess for constructing a DMOS device.

FIGS. 4A-4U are device cross-sectional views illustrating in detail anexample fabrication process used for fabricating an MOS device.

FIGS. 5A-6B illustrates additional alternative embodiments offabrication steps.

FIGS. 7-10 illustrate optional modifications to the fabrication processthat are used in some embodiments to further enhance device performance.

DETAILED DESCRIPTION

The invention can be implemented in numerous ways, including as aprocess, an apparatus, a system, a composition of matter, a computerreadable medium such as a computer readable storage medium or a computernetwork wherein program instructions are sent over optical orcommunication links. In this specification, these implementations, orany other form that the invention may take, may be referred to astechniques. A component such as a processor or a memory described asbeing configured to perform a task includes both a general componentthat is temporarily configured to perform the task at a given time or aspecific component that is manufactured to perform the task. In general,the order of the steps of disclosed processes may be altered within thescope of the invention.

A detailed description of one or more embodiments of the invention isprovided below along with accompanying figures that illustrate theprinciples of the invention. The invention is described in connectionwith such embodiments, but the invention is not limited to anyembodiment. The scope of the invention is limited only by the claims andthe invention encompasses numerous alternatives, modifications andequivalents. Numerous specific details are set forth in the followingdescription in order to provide a thorough understanding of theinvention. These details are provided for the purpose of example and theinvention may be practiced according to the claims without some or allof these specific details. For the purpose of clarity, technicalmaterial that is known in the technical fields related to the inventionhas not been described in detail so that the invention is notunnecessarily obscured.

A metal oxide semiconductor (MOS) device and its fabrication aredescribed. For the purpose of example, N-channel devices with source anddrain made of N-type material and body made of P-type material arediscussed in detail throughout this specification. The techniques andstructures disclosed herein are also applicable to P-channel devices.

FIGS. 1A-1F illustrate several double-diffused metal oxide semiconductor(DMOS) device embodiments. FIG. 1A is a cross sectional view of anembodiment of a DMOS device. In this example, device 100 includes adrain that is formed on the back of an N⁺-type semiconductor substrate103. The drain region extends into an epitaxial (epi) layer 104 ofN⁻-type semiconductor that overlays substrate 103. Gate trenches such as111, 113, and 115 are etched in epi layer 104. A gate oxide layer 121 isformed inside the gate trenches. Gates 131, 133 and 135 are disposedinside gate trenches 111, 113 and 115, respectively, and are insulatedfrom the epi layer by the oxide layer. The gates are made of aconductive material such as polycrystalline silicon (poly) and the oxidelayer is made of an insulating material such as thermal oxide.Specifically gate trench 111 is located in a termination region disposedwith a gate runner 131 for connection to gate contact metal. For thatpurpose gate runner trench 111 may be wider and deeper compared toactive gate trenches 113 and 115. Further the spacing between the gaterunner trench 111 from the active trench next to it, in this case trench113, may be larger than the spacing between the active gate trenches 113and 115.

Source regions 150 a-d are embedded in body regions 140 a-d,respectively. The source regions extend downward from the top surface ofthe body into the body itself. While body regions are implanted alongside of all gate trenches, source regions are only implanted next toactive gate trenches and not gate runner trenches. In the embodimentshown, gates such as 133 have a gate top surface that extendssubstantially above the top surface of the body where the source isembedded. Such a configuration guarantees the overlap of the gate andthe source, allowing the source region to be shallower than the sourceregion of a device with a recessed gate, and increases device efficiencyand performance. The amount by which the gate poly top surface extendsabove the source-body junction may vary for different embodiments. Insome embodiments, the gates of the device do not extend above the topsurface of the source/body region, rather recess from the top surface ofthe source/body region.

During operation, the drain region and the body regions together act asa diode, referred to as the body diode. A dielectric material layer 160is disposed over the gate to insulate the gate from source-body contact.The dielectric material forms insulating regions such as 160 a-c on topof the gates as well as on top of the body and source regions.Appropriate dielectric materials include thermal oxide, low temperatureoxide (LTO), boro-phospho-silicate glass (BPSG), etc.

A number of contact trenches 112 a-b are formed between the active gatetrenches near the source and body regions. These trenches are referredto as active region contact trenches since the trenches are adjacent tothe device's active region that is formed by the source and bodyregions. For example, contact trench 112 a extends through the sourceand the body, forming source regions 150 a-b and body regions 140 a-badjacent to the trench. In contrast, trench 117, which is formed on topof gate runner 131, is not located next to an active region, andtherefore is not an active region contact trench. Trench 117 is referredto as a gate contact trench or gate runner trench since a metal layer172 a connected to the gate signal is deposited within the trench. Gatesignal is fed to active gates 133 and 135 through interconnectionsbetween trenches 111, 113 and 115 in the third dimension (not shown).Metal layer 172 a is separated from metal layer 172 b, which connects tosource and body regions through contact trenches 112 a-b to supply apower source. In the example shown, the active region contact trenchesand gate contact trench have approximately the same depth.

Device 100 has active region contact trenches 112 a-b that are shallowerthan the body. This configuration provides good breakdowncharacteristics as well as lower resistance and leakage current.Additionally, since the active contact trenches and gate contact trenchare formed using a one step process therefore have the same depth,having active contact trenches that are shallower than the body preventsthe gate runner such as 131 from being penetrated by the gate contacttrench.

In the example shown, the FET channel is formed along the active regiongate trench sidewall between the source/body and body/drain junctions.In a device with a short channel region, as the voltage between thesource and the drain increases, the depletion region expands and mayeventually reach the source junction. This phenomenon, referred to aspunch through, limits the extent to which the channel may be shortened.In some embodiments, to prevent punch through, regions such as 170 a-dalong the walls of the active region contact trench are heavily dopedwith P type material to form P⁺-type regions. The P⁺-type regionsprevent the depletion region from encroaching upon the source region.Thus, these implants are sometimes referred to as anti-punch throughimplants or punch through prevention implants. In some embodiments, toachieve pronounced anti-punch through effects, the P⁺ regions aredisposed as close as possible to the channel region and/or as close asit is allowed by manufacturing alignment capability and P⁺ sidewalldopant penetration control. In some embodiments, the misalignmentbetween the trench contact and the trench is minimized by self-aligningthe contact, and the trench contact is placed as closely centeredbetween the trenches as possible. These structural enhancements allowthe channel to be shortened such that the net charge in the channel perunit area is well below the minimum charge needed to prevent punchthrough in an ideal unprotected structure. In addition to improving bodycontact resistance, the anti-punch through implants also makes itpossible to construct very shallow trench short-channel devices. In theembodiment shown, contact trenches 112 a-b are shallower than bodyregions 140 a-d and do not extend all the way through the body regions.The device's on-resistance R_(dson) as well as the gate capacitance arereduced.

A conductive material is disposed in contact trenches 112 a-b as well asgate trench 117 to form contact electrodes. In the active region, sincethe punch-through implants are disposed along the sidewalls of thecontact trenches but not along the bottoms of the contact trenches, thecontact electrodes are in contact with N⁻ drain region 104. Together,the contact electrodes and the drain region form Schottky diodes thatare in parallel with the body diode. The Schottky diodes reduce the bodydiode forward drop and minimize the stored charge, making the MOSFETmore efficient. A single metal that is capable of simultaneously forminga Schottky contact to the N⁻ drain and forming good Ohmic contact to theP⁺ body and N⁺ source is used to form electrodes 180 a-b. Metals such astitanium (Ti), platinum (Pt), palladium (Pd), tungsten (W) or any otherappropriate material may be used. In some embodiments, metal layer 172is made of aluminum (Al) or made of a Ti/TiN/Al stack.

The leakage current of the Schottky diode is related to the Schottkybarrier height. As the barrier height increases, the leakage currentdecreases, and the forward drop voltage also increases. In the exampleshown, optional Schottky barrier controlling layers 190 a-b (also knownas Shannon layers) are formed below the contact electrode, by implantingthin layers of dopants around the bottoms of active region trenches 112a-b. The dopants have opposite polarity as the epi layer and are of Ptype in this example. The Shannon implant is shallow and low dosage;therefore, it is completely depleted regardless of bias. The Schottkybarrier controlling layer is used to control the Schottky barrierheight, thus allowing for better control over the leakage current andimproving the reverse recovery characteristics of the Schottky diode.Details of the formation of the Schottky barrier controlling layer aredescribed below.

FIG. 1B is a cross sectional view of another embodiment of a DMOSdevice. Device 102 also includes Schottky barrier controlling layers 190a-b around the bottoms of the active region contact trenches. In thisexample, the depth of gate contact trench 117 is different from that ofactive region contact trenches 112-b. The active region contact trenchesare deeper than body regions 140 a-d and extend beyond the body regions.Since the active contact trench is deeper, it provides more area formaking Ohmic contact along the sidewalls and results in better unclampedinductive switching (UIS) capability. Furthermore, by making the gatecontact trench shallower than the active contact trenches, it isunlikely that the gate contact trench would penetrate the gate runnerpoly during the etching process, which is useful for devices withrelatively shallow gate polys (such as devices fabricated usingprocesses that result in gate polys that do not extend above the bodytop surface).

FIG. 1C is another embodiment of a DMOS device. In this example, gatecontact trench 117 and active region contact trenches 112 a-b havedifferent depths. Further, the depth of each of the active regioncontact trenches is non-uniform since the trench depth varies in thedirection parallel to the substrate surface. As will be described inmore detail below, the active region contact trenches are formed using a2-step process, resulting in a first contact opening (e.g. 112 a-b) thatis wider than a second contact opening (e.g. 119 a-b). The shape of theprofile of the active region contact trench allows for greater Ohmiccontact area and better punch-through prevention by anti-punch throughimplants 170 a-d, and improves the device's UIS capability. Shannonimplants distribute around the sidewall and bottom of the second contactopening, forming a Schottky barrier controlling layer 190 a-b.

FIGS. 1D-1F illustrate embodiments of DMOS devices with integrated lowinjection body diode. Devices 106, 108, and 110 have active regioncontact trenches that are shallower than the body regions. In someembodiments, a body layer of approximately 0.01˜0.5 μm separates thebottom of the active region trench from the epi, forming a low injectiondiode below the body/drain junction. The thickness and the doping levelof the thin body layer, which lies between the active region contacttrench and the drain, are adjusted so that in reverse bias this thinbody layer is almost completely depleted while in forward bias the bodylayer is not depleted. The integration of such a low injection diode indevices 106, 108 and 110 provide performance improvement over theregular body diode as carrier has been greatly reduced. With propercontrol of thin body layer a low injection body diode may providecomparable performance as the Schottky diode, with the advantage ofsimplified process as formation of Schottky barrier controlling layercan be omitted.

FIG. 2 is a diagram illustrating a buck converter circuit example. Inthis example, circuit 200 is shown to employ a high side FET device 201and a low side FET device 207. High side device 201 includes atransistor 202 and a body diode 204. Low side device 207 can beimplemented using devices such as 100, 102, or 104 shown in FIGS. 1A-1F.Device 207 includes a transistor 208, a body diode 210 and a Schottkydiode 212. The load includes an inductor 214, a capacitor 216 and aresistor 218. During normal operation, device 201 is turned on totransfer power from the input source to the load. This causes thecurrent to ramp up in the inductor. When device 201 is turned off, theinductor current still flows and commutates to body diode 210 of device207. After a short delay, the control circuit turns on device 207, whichturns on the channel of transistor 208 and dramatically reduces theforward drop across the drain-source terminals of device 208. WithoutSchottky diode 212, the body diode conduction loss and the losses fromremoving the stored charge in body diode 210 of device 207 can besubstantial. However, if Schottky diode 212 is built into device 207 andif the Schottky diode has a low forward drop, the conduction loss isgreatly reduced. Since the low forward drop across the Schottky diode islower than the junction drop of the body diode, no stored charge isinjected while the Schottky diode conducts, further improving the lossesrelated to diode recovery.

FIG. 3 is a flowchart illustrating an embodiment of a fabricationprocess for constructing a DMOS device. At 302, gate trenches are formedin the epi layer overlaying the semiconductor substrate. At 304, gatematerial is deposited in the gate trenches. At 306 and 308, the body andthe source are formed. At 310, contact trenches are formed. As will bediscussed in further detail below, in some embodiments, the activeregion contact trenches and the gate region trenches are formed in asingle step; in some embodiments, the trenches are formed in multiplesteps to achieve different depth. At 312, contact electrodes aredisposed within the contact trenches. Process 300 and its steps can bemodified to produce different embodiments of MOS devices such as 102-110shown in FIGS. 1A-1F.

FIGS. 4A-4V are device cross-sectional views illustrating in detail anexample fabrication process used for fabricating an MOS device. In thisexample, an N type substrate (i.e., an N⁺ silicon wafer with an N⁻ epilayer grown on it) is used as the drain of the device.

FIGS. 4A-4J shows the formation of the gate. In FIG. 4A, a SiO₂ layer402 is formed on N type substrate 400 by deposition or thermaloxidation. The thickness of the silicon oxide ranges from 100 Å to 30000Å in various embodiments. Other thicknesses can be used. The thicknessis adjusted depending on the desired height of the gate. A photoresistlayer 404 is spun on top of the oxide layer and patterned using a trenchmask.

In FIG. 4B, the SiO₂ in the exposed areas is removed, leaving a SiO₂hard mask 410 for silicon etching. In FIG. 4C, the silicon is etchedanisotropically, leaving trenches such as 420. The gate material isdeposited in the trenches. Gates that are later formed within the trenchhave sides that are substantially perpendicular to the top surface ofthe substrate. In FIG. 4D, SiO₂ hard mask 410 is etched back by anappropriate amount so that the trench walls remain approximately alignedwith the edge of the hard mask after later etching steps. SiO₂ is themask material used in this embodiment because etching using a SiO₂ hardmask leaves relatively straight trench walls that mutually align withthe sides of the mask. Other material may be used as appropriate.Certain other types of material traditionally used for hard masketching, such as Si₃N₄, may leave the etched trench walls with acurvature that is less desirable for gate formation in the followingsteps.

In FIG. 4E, the substrate is etched isotropically to round out thebottoms of the trenches. The trench is approximately between 0.5-2.5 μmdeep and approximately between 0.2-1.5 μm wide in some embodiments;other dimensions can also be used. To provide a smooth surface forgrowing gate dielectric material, a sacrificial layer of SiO₂ 430 isgrown in the trenches. This layer is then removed by the process of wetetching. In FIG. 4G, a layer of SiO₂ 432 is grown thermally in thetrenches as dielectric material.

In FIG. 4H, poly 440 is deposited to fill up the trenches. In this case,the poly is doped to obtain the appropriate gate resistance. In someembodiments, doping takes place as the poly layer is deposited (insitu). In some embodiments, the poly is doped after the deposition. InFIG. 4I, the poly layer on top of the SiO₂ is etched back to form gatessuch as 442. At this point, top surface 444 of the gate is stillrecessed relative to top surface 448 of the SiO₂; however, top surface444 of the gate may be higher than top layer 446 of the silicon,depending on the thickness of hard mask layer 410. In some embodiments,no mask is used in poly etch back. In some embodiments, a mask is usedin poly etch back to eliminate the use of an additional mask in thefollowing body implanting process. In FIG. 4J, the SiO₂ hard mask isremoved. In some embodiments, dry etch is used for hard mask removal.The etching process stops when the top silicon surface is encountered,leaving the poly gate extending beyond the substrate surface wheresource and body dopants will be implanted. In some embodiments, the gateextends beyond the substrate surface by approximately between 300 Å to20000 Å. Other values can also be used. A SiO₂ hard mask is used inthese embodiments since it provides the desired amount of gate extensionbeyond the Si surface in a controllable fashion. A screen oxide may thenbe grown across the wafer. The above processing steps may be simplifiedfor fabricating devices with recessed gate poly. For example, in someembodiments a photoresist mask or a very thin SiO₂ hard mask is usedduring trench formation, and thus the resulting gate poly does notextend beyond the Si surface.

FIGS. 4K-4N illustrate the formation of the source and the body. In FIG.4K, a photoresist layer 450 is patterned on the body surface using abody mask. The unmasked regions are implanted with body dopants. Dopantssuch as Boron ions are implanted. In some embodiments that are not shownhere, the body implant is carried out without body block 450, forming acontinuous body region between active trenches. In FIG. 4L, thephotoresist is removed and the wafer is heated to thermally diffuse theimplanted body dopants via a process sometimes referred to as bodydrive. Body regions 460 a-d are then formed. In some embodiments, theenergy used for implanting the body dopants is approximately between30˜600 keV, the dose is approximately between 5e12-4e13 ions/cm², andthe resulting final body depth is approximately between 0.3-2.4 μm.Different depths can be achieved by varying factors including theimplant energy, dose and diffusion temperature. An oxide layer 462 isformed during the diffusion process.

In FIG. 4M, a photoresist layer 464 is patterned using a source mask. Inthe embodiment shown, source mask 464 does not block any area betweenactive trenches. In some embodiments, source mask 464 also blocks acenter area of between active trenches (not shown). The unmasked region466 is implanted with source dopants. In this example, arsenic ionspenetrate the silicon in the unmasked areas to form N⁺ type source. Insome embodiments, the energy used for implanting the source dopants isapproximately between 10˜100 keV, the dose is approximately between1e15-1e16 ions/cm², and the resulting source depth is approximatelybetween 0.05-0.5 μm. Further depth reduction can be achieved by varyingfactors such as the doping energy and dose. Other implant processes mayalso be used as appropriate. In FIG. 4N, the photoresist is removed andthe wafer is heated to thermally diffuse the implanted source dopantsvia a source drive process. A dielectric (e.g. BPSG) layer 465 isdisposed on the top surface of the device after source drive, andoptionally densified in some embodiments.

FIGS. 4O-4T illustrate the formation of the contact trench and variousimplants along the contact trench. In FIG. 4O, a photoresist layer 472is deposited on the dielectric layer and patterned using a contact mask.A first contact etch is performed to form trenches 468 and 470. In someembodiments, the depth of the first contact trench is between 0.2-2.5μm.

In FIG. 4P, the photoresist layer is removed, and the area around thebottom of trench 470 is bombarded with implant ions to form apunch-through prevention layer. Boron ions with a dose of approximately1-5e15 ions/cm² are used in some embodiments. The implant energy isapproximately 10-60 kEv. In some embodiments, BF₂ ions with a dose ofapproximately 1-5e15 ions/cm² and implant energy of 40-100 kEv are used.In some embodiments, both BF₂ and Boron are implanted to form the formthe punch-through prevention layer. The implantation tilt isapproximately between 0-45°. In FIG. 4Q, the implant is thermallydiffused.

In FIG. 4R, a second contact etch takes place. Since the etching processdoes not affect the dielectric layer, the second contact etch does notrequire an extra mask. The depth of the trenches is increased by 0.2-0.5μm in some embodiments. The punch-through prevention layer is etchedthrough, leaving the anti-punch-through implants 474 a-b along thetrench wall. In FIG. 4S, a low dose shallow P type Schottky barriercontrolling layer 476 is formed using ion implantation. In someembodiments, Boron or BF2 with a dosage between 2e11-3e13 ions/cm² andimplant energy between 10-100 kEv are used. In FIG. 4T, the Schottkybarrier controlling layer is activated by thermal diffusion. Incomparison to the anti-punch through implant, the Schottky barriercontrolling layer requires a lower dose and thus results in a lowerdoping and thinner layer of implant. In some embodiments, the Schottkybarrier controlling layer is approximately 0.01˜0.05 μm thick. TheSchottky barrier controlling layer can adjust the barrier height becausethe implant adjusts the surface energy between the contact electrode andthe semiconductor.

In FIG. 4U, completed device 490 is shown. Metal layer 478 is deposited,etched where appropriate, and annealed. Passivation openings are madeafter a passivation layer 480 is deposited. Additional steps required tocomplete the fabrication such as wafer grinding and back metaldeposition are also performed.

Alternative processes may be used. For example, to fabricate devices106-110 shown in FIGS. 1D-1F, the body implant process shown in FIG. 4Kis modified and no body block in active area. Body dopants are directlyimplanted, blanketing the exposed areas and forming continuous bodyregions between gates. During contact etch, trenches are etched to adepth that is shallower than the bottom of the body region, leaving abody layer below the contact trench. Alternatively, active contacttrench may be etched just through the body to expose epi drain regionfollowed by an additional body dopant implant with well controlledenergy and dopant to form a thin body layer through the contact trenchsidewall and bottom.

In some embodiments, to form the Schottky barrier controlling layer, anarrow bandgap material such as SiGe is deposited by chemical vapordeposition (CVD) to form a layer on the top surface of an epitaxiallayer. The thickness of narrow bandgap material layer is in the rangefrom 100 Å to 1000 Å in some embodiments. For example, a 200 Å siliconrich SiGe layer is used in some embodiments. In some embodiments, thesilicon rich SiGe layer comprises 80% Si and 20% Ge. In someembodiments, the narrow bandgap material layer is in-situ doped with Ntype dopant at a concentration between 2e17 to 2e18/cm³. A lowtemperature oxide layer is then deposited over the narrow bandgap layer,and patterned to form a hard mask for dry etching trenches into theepitaxial layer. The hard mask protects portions of the narrow bandgaplayer underneath during the dry etching process.

FIGS. 5A-6B illustrates additional alternative embodiments offabrication steps. For example, FIG. 5A follows the punch-throughprevention layer diffusion (see FIG. 4Q). A photoresist layer 502 ispatterned using a second contact mask to block gate trench 504. In FIG.5B, a second etch takes place to increase the depth of active regioncontact trench 506. The photoresist is then removed, and Schottkybarrier controlling layer is implanted in a manner similar to FIGS. 4Sand 4T. Additional finishing steps including metal deposition andpassivation still apply (see FIG. 4U). The resulting device is similarto device 102 of FIG. 1B, where the gate trench has a different depththan the active region contact trench. By using a separate mask for thesecond contact trench etching to achieve different gate trench andactive region contact trench depths, the gate trench contact can be madeshallower and alleviate concerns over puncturing the gate poly duringetching. Thus, the process is often used to fabricate devices with shortgate polys, including embodiments with gate polys that do not extendabove the substrate surface.

FIG. 6A also follows the punch-through prevention layer diffusion (seeFIG. 4Q). A photoresist layer 602 is patterned using a second contactmask to block gate trench 604 and to form a contact opening over activeregion contact trench 606 that is smaller than the contact opening fromthe first etch. In FIG. 6B, a second contact etch is performed to form adeeper, narrower trench portion 608. The photo resist is removed andremaining steps from FIGS. 4S-4U apply. The resulting device is similarto 103 of FIG. 1C.

FIGS. 7-10 illustrate optional modifications to the fabrication processthat are used in some embodiments to further enhance device performance.

The optional modification shown in FIG. 7 may take place after the gateshave been formed (FIG. 4G) and prior to applying the body block mask(FIG. 4K). A blanket implant 702 having the opposite polarity as theepitaxial layer is deposited throughout the epi. In some embodiments, ahigh energy, low dose of Boron (5e11˜1e13, 200˜600 keV) is used to formblanket implant 702 before the formation of the main body implant. Theblanket implant is used to adjust epi profile without resulting inpolarity changing in the epi. The blanket implant changes the bodyprofile in the body bottom region, and enhances the breakdown voltagewithout noticeably increasing R_(dson).

The optional modification shown in FIG. 8 may take place after theShannon implant has been deposited (FIG. 4S) but prior to its activation(FIG. 4T). An epitaxial layer profile tuning implant is implanted underthe active region contact trench. The epitaxial layer profile tuningimplant has the opposite polarity as the epi. In some embodiments a highenergy, low dose of Boron or BF₂ (e.g., 5e11˜1e13, 60˜300 keV) is usedfor the implant. The implant tunes the epi profile without changing theepi polarity, and enhances the breakdown voltage.

The optional modification shown in FIG. 9 may take place after theShannon implant has been deposited (FIG. 4S) but prior to its activation(FIG. 4T). A high energy, medium dose of Boron (1e12˜5e13, 60˜300 keV)is implanted to form a P type island 902 that is located in the n-typeepi under contact trench and disconnected from the body region. Thefloating P type island also enhances breakdown voltage.

The optional modification shown in FIG. 10 may take place after thecontact trench has been made (FIG. 4O) and prior to making the Shannonimplant (FIG. 4P). Since sharp corners tend to accumulate electriccharges, creates high electric fields, and lower breakdown voltage,corners of the trench bottom 1002 a-b are rounded to reduce chargeaccumulation and improve breakdown voltage.

Although the foregoing embodiments have been described in some detailfor purposes of clarity of understanding, the invention is not limitedto the details provided. There are many alternative ways of implementingthe invention. The disclosed embodiments are illustrative and notrestrictive.

What is claimed is:
 1. A method of fabricating a semiconductor device,comprising: forming a gate trench in an epitaxial layer overlaying asemiconductor substrate; depositing gate material in the gate trench;forming a body; forming a source; forming an active region contacttrench that extends through the source and the body into a drain;forming a Schottky barrier controlling layer in the epitaxial layer inbottom region of the active region contact trench, wherein the Schottkybarrier controlling layer does not extend substantially into the body;and disposing a contact electrode within the active region contacttrench.
 2. The method of claim 1, wherein forming the Schottky barriercontrolling layer includes depositing a layer of material doped withopposite polarity dopant as the epitaxial layer in the epitaxial layerin the bottom region of the active region contact trench.
 3. The methodof claim 1, wherein the gate trench is a first gate trench; and themethod further comprising forming a gate region, including by: forming asecond gate trench extending into the epitaxial layer; forming a secondgate disposed in the second gate trench; and forming a gate contacttrench formed within the second gate.
 4. The method of claim 3, whereinthe gate contact trench and the active region contact trench haveapproximately the same depth.
 5. The method of claim 1, furthercomprising depositing a blanket implant deposited throughout theepitaxial layer, wherein the blanket implant has opposite polarity asthe epitaxial layer.
 6. The method of claim 1, further comprisingforming an epitaxial layer profile tuning implant deposited under theactive region contact trench.
 7. The method of claim 1, furthercomprising forming an island region below the active region contacttrench, wherein the island region has opposite polarity as the epitaxiallayer.
 8. The method of claim 1, wherein the gate material is formed toextend above the body top surface.